Cosmetically... there seems to be a slight difference in the external profile where the front of the receiver meets the barrel, or at least it has seemed that way on every firearm I Knew was a P90 and not a converted PS90.
The P90 has a right-angle stepped profile and the PS90 has a 45-degree angle.

versus

Then again, I could just be imagining differences that don't really exist.

I have a gen 1 ps90 and my friend has his class 3 and he has a p90. We did an experiment with interchanging them and shooting them. They are in fact interchangable. They also state that SS197 will not run on fully automatic fire, there are so many stupud rumors out there. Everything swaps out between the 2.

On the top example, just above the left-most screw, the polymer frame makes a 45-degree angle to join the metal receiver.

On the bottom example, in the same place (except now it's the right-most screw), the polymer frame continues straight up until it reaches the level of the metal receiver and then makes a 90-degree turn to join.

I'm not saying it's a definitive way to tell the two apart, but so far on every example where I've been assured that what is presented is an actual P90 and not a shortened PS90 it's been accurate.

Which is "...right from FNH's website" the photo you loaded right from Photobucket?

If I were to say something was 'right from FNH's website' then I'd probably post something like
or

But, that only shows the exact damn thing I was saying about there being a cosmetic difference between a P90 and a shortened PS90. I gather that's something we might not want to do?

By the bye, I'm not saying you can't slap the receiver halves of a P90 in place of a PS90, nor the other way 'round. I'm pointing out that there is a difference between two receiver halves and that so far, the difference is consistent between actual, confirmed OEM P90's and shortened PS90's. A c-o-s-m-e-t-i-c difference.

My ps90 does not have that 45 degree angle it is straight just like the p90. Due to the different models/years they have changed there stocks in different ways but back to the THREAD POST. The two can swap out without any problems.

What was once an LEO P90 was destroyed according to ATF rules, meaning the receiver was cut up. It leave the remaining parts for sale. Have I missed something? If one were to wrap those parts around their PS90 receiver, would they have a PS90 that functions like a P90? Something tells me that it might look legal, but would still get you in trouble.